Optical characterization of a medical condition of a patient is currently an area of growth, partly due to the increasing number of possible medical conditions that are detectable by emerging optical technologies. In particular, early detection by optical methods of for example cancer may facilitate an opportunity for improved detection giving rising to an increased chance of survival for the patient. When medical relevant information is available from very small tissue volume, even pre-malignant changes in tissue morphology and physiology may become distinguishable.
More than 90% of all cancers are epithelial of origin. Body surfaces are mostly covered with a thin layer of epithelial tissue. This epithelial tissue layers of various organs have thickness ranging from less than 10 micron in simple (single layer) squamous epithelia to several hundred micron in stratified (multiple cell layers) epithelia. Below the epithelia layers various other tissue layers are present such as connective tissue, inflammatory cells, neurovascular structures etc. Since the penetration depth of light is, in general, much larger than that of the epithelial layer the backscattered light from the tissue contains information of the change in the epithelial layers superimposed on a large background signal arising form the deeper layered tissues. This makes it difficult to extract the relevant information from this signal directly. To solve this problem a method is needed in which the signal from the epithelial layer can be disentangled from the signal due to the deeper layers i.e. the background signal.
V. Backman et al. disclosed a solution to this problem in IEEE J. Selected Topics Quantum Electron., Vol. 5, No 4, July/August 1999, p. 1019. In the method by Backman et al., polarized light is used to illuminate the tissue. Thereafter, they detect the scattered light having the same polarization and the orthogonal polarization separately by using a polarizing beam splitter and two separate detectors. Since the signal coming from the epithelial layer will be scattered typically only once the polarization will be significantly preserved. The scattered light coming from the deeper layers, being multiple scattered, will loose the original polarization information and will become isotropically distributed whereby the original polarization is lost. By subtracting both signals from each other one can remove the background signal from the desired signal being backscattered from the epithelial layer.
A drawback of the method of Backman et al. is that there will normally still be single scattered photons coming from the layer deeper than the epithelial layer, these deeper layer may thus negatively influence the desired signal. Furthermore, because a large background signal is removed from the smaller actual signal, a significant amount of noise will be present in the final signal, which limits the measurement accuracy. This in turn limits the detection limit with regard to how early a cancer in the tissue can be detected. Additionally, if a patient is under temporal influence of a substance, e.g. a pharmaceutical drug, the optical properties of the epithelial layers may change in response to the said substance, thereby making the reliability of an optical assessment of a patient with this method even lower.
Hence, an improved optical device would be advantageous, and in particular a more efficient and/or reliable optical device would be advantageous.